The present invention relates to the field of disc drive data storage devices. More particularly, this invention relates to a method and an apparatus for managing a seek mode in a disc drive with nonlinear pivot friction.
important device in any computer system is a data storage device. Computer systems have many different places where data can be stored. One place for storing massive amounts of data and instructions is a disc drive. The disc drive has one or more discs, each with two surfaces on which data is stored. The surfaces are coated with a magnetizable medium that is magnetized in alternate directions to store the data and instructions. The surfaces are computer-readable mediums holding computer-readable data and computer-readable and computer-executable instructions. The discs are mounted on a hub of a spindle motor for rotation at an approximately constant high speed during the operation of the disc drive. An actuator assembly in the disc drive moves transducers to various locations relative to the discs while the discs are rotating, and electrical circuitry is used to write data to and read data from the discs through the transducers. Data and instructions are stored on one or both of the surfaces of each disc. The disc drive also includes circuitry for encoding data and instructions written to the discs and for decoding data and instructions read from the discs. A microprocessor controls most operations of the disc drive, such as transmitting information including instructions or data read from the discs back to a requesting computer and receiving data or information from the requesting computer for writing to the discs.
Each transducer is typically located on a small ceramic block, also referred to as a slider, that is aerodynamically shaped so that it flies over a surface of one of the discs. When the disc rotates, air flow is induced between the slider and the disc, causing air pressure which forces the slider away from the disc. The slider is also attached to a load spring that produces a force on the slider directed toward the disc. The forces on the slider equilibrate such that the slider flies over the surface of the disc at a desired fly height.
Information representative of data or instructions is stored in tracks on the surfaces of the discs. In some disc drives, information is stored in a multiplicity of concentric circular tracks on the surface of each disc. In other disc drives, information is stored in a single track that forms a continuous spiral on each side of the discs. A transducer carried by a slider is positioned over a track on a surface of one of the discs to write information to or read information from the track. Once the operation is complete, the transducer may be controlled to move to a new, target track, to write information to or read information from the target track. The movement takes place in the following modes. The transducer is moved radially across the surface of the disc in a seek mode to position it near the target track. The transducer is then positioned over the target track during a track-and-follow mode, also called a tracking mode, to read or write the information stored in the target track. Servo information is read from the target track by the transducer, and a feedback control system determines a position error signal from the servo information. If the transducer is not in a correct position, it is moved to a desired position over the target track in response to the position error signal.
Each slider is supported by the actuator assembly which is controlled to position the transducer in the slider over the surface of one of the discs. Each slider is attached to a load spring supported by an arm. The arms in the actuator assembly are rotatably mounted to an actuator shaft through bearings and are rotated about the actuator shaft by a voice coil motor to move the transducers over the surfaces of the discs. The bearings and the actuator shaft are also called a pivot. The voice coil motor includes a voice coil mounted to the actuator assembly opposite to the arms. The voice coil is immersed in a magnetic field of an array of permanent magnets placed adjacent to the actuator assembly. The feedback control system applies current to the voice coil in a first direction to generate an electromagnetic field that interacts with the magnetic field of the magnets. The interaction of the magnetic fields applies a torque to the voice coil to rotate the actuator assembly about the pivot, and the actuator assembly is accelerated to move the transducer to a new position. The feedback control system may then apply current to the voice coil in a direction opposite to the first direction to apply an opposite torque on the actuator assembly. The opposite torque may be used to decelerate the actuator assembly and position the transducer over a target track. The opposite torque may also be used to accelerate the actuator assembly to a different position.
The actuator assembly is subject to different amounts of torque, also called bias, at different positions relative to the discs. The bias is a result of several factors, including spring action operating on the actuator assembly due to a flexible cable connecting electrical circuitry in the actuator assembly to other electrical circuits in the disc drive, gravity, windage acting on the arms, friction in the pivot, the direction of a preceding rotation of the actuator assembly, and other factors. The bias is nonlinear, in part because the friction in the pivot is nonlinear. The current applied to the voice coil to maintain the position of one of the sliders in a tracking operation is augmented to overcome the bias.
During both the seek and track-and-follow modes it has become common to balance the bias described above with a bias current applied to the voice coil to induce a torque on the actuator assembly that is opposed to the bias. The bias current is selected from a bias table that has bias current entries for each position of the actuator assembly. The bias current entries are partitioned in the bias table according to tracks or zones of adjacent tracks on the discs over which the sliders may be positioned.
Disc drives are being produced with increasing track densities and decreasing access times. Feedback control systems in modem disc drives must move the sliders to the correct position in a very short period of time. Incorrect bias current entries in the bias table can cause seek errors for a feedback control system. A seek error occurs when the feedback control system does not move the slider close enough to the target track in the seek mode to begin tracking. The seek mode must be repeated in the event of a seek error. A seek error rate refers to the number of seek errors that occur during a period of operation of the disc drive. A disc drive that exhibits a high seek error rate performs poorly because its average access time is increased. At an extreme, a disc drive may be inoperable because of a high seek error rate.
The bias current entries in the bias table are updated with output from a bias estimator, which is generated from the position error signals. However, the nonlinearities in the bias described above, including the nonlinearity of the friction in the bearings, leads to incorrect updates of the bias current entries and an increase in the seek error rate. There remains a need for a disc drive that updates bias current entries in a bias table more accurately to minimize seek errors. Such a disc drive should update the bias current entries more accurately in view of the nonlinear bias that an actuator assembly is subject to, such as nonlinear pivot friction. Such a disc drive would have improved performance because of a reduced seek error rate.
According to one embodiment of the present invention, a disc drive system includes a disc and a transducer supported by an actuator assembly that is accelerated by controlling current in a voice coil in the actuator assembly. The disc drive system also includes a control circuit operatively configured to control a position of the transducer over a present track on the disc in a track-and-follow mode, to generate an estimated bias current to be applied to the voice coil to balance a bias on the actuator assembly when the transducer is over the present track, to start a movement of the transducer toward a target track in a seek mode, to enter the estimated bias current into a bias table if an immediately preceding movement of the transducer in the seek mode was longer than a seek length boundary, and to apply a bias current to the voice coil calculated based on a bias current entry in the bias table during the seek mode. The seek length boundary is generated to indicate seek lengths shorter than the seek length boundary for which nonlinear friction in a pivot in the actuator assembly is more substantial and seek lengths equal to or longer than the seek length boundary for which nonlinear friction in the pivot is less substantial.
Advantageously, in the disc drive system according to the above-described embodiment of the present invention, the bias table is updated with the estimated bias current only if an immediately preceding movement of the transducer in the seek mode was longer than a seek length boundary. The bias table is not changed following movements of the transducer that are shorter than the seek length boundary, for which nonlinear friction in the pivot is more substantial. The bias current entries in the bias table are updated more accurately in view of nonlinear pivot friction to minimize seek errors. The disc drive system therefore has a reduced seek error rate.